Automobiles, industrial machines, and similar devices contain many rotating portions, which are always equipped with sliding parts. For example, automobiles are equipped with sliding parts such as bearings in portions for receiving a rotating shaft, gear pumps for hydraulic equipment are equipped with sliding parts such as side plates for restraining the side surfaces of gears, and compressors are equipped with sliding parts such as swash plates.
When machines having sliding parts installed therein malfunction and become expensive to repair or become old and no longer function as desired, they are discarded. In order to conserve resources, many of the materials constituting machines are recovered and reused. However, sliding parts installed in machines have been disposed of by burial. This is because in many sliding parts, a steel plate which forms a backing plate and a sliding material cannot be easily separated from each other. In order to increase the mechanical strength of sliding parts, a sliding material and a steel plate are metallically bonded to each other, namely, the metal in the sliding material and the steel plate are metallically bonded to each other with the metal atoms of each one penetrating the other. Therefore, the sliding material and the steel plate cannot be separated from each other and recovered. Accordingly, even if it is attempted to melt sliding parts having a large proportion of iron and recover the iron, a large amount of other components are intermixed with the iron, and it cannot be used as iron resources. Thus, many sliding parts have been disposed of by burial as industrial waste.
Many conventional sliding materials were made of lead bronze (LBC3) in which Pb is added to a Cu alloy. Lead bronze has Pb dispersed in a Cu—Sn alloy matrix. The hard Cu—Sn alloy matrix supports an opposing member without wearing, while the Pb spreads as a thin layer on the surface of the matrix and performs the function of a lubricating oil to provide good sliding properties. Thus, lead bronze is inexpensive and has suitable sliding properties, and it has been used in various types of sliding parts from long in the past.
When Pb is dispersed in a sliding material in this manner, it provides excellent sliding properties. Therefore, it has been conceived of using even more Pb in sliding materials, and the surfaces of such sliding parts have been provided with an overlay in the form of Pb alloy plating. Overlays include Pb alloy plating on the surface of a sliding part, and overlays in which a copper alloy powder is sintered to form a porous portion and a Pb alloy is melted and impregnated into the porous portion. See JP S56-16603A and JP S49-54211A, for example.
However, when sliding parts using lead bronze or sliding parts overlaid with a Pb alloy are disposed of by burial and are contacted by acid rain, Pb in the sliding material is dissolved out and pollutes underground water. If this underground water containing Pb is drunk for long periods of time by humans or livestock, the Pb accumulates in the body, and it is said to eventually cause lead poisoning. Therefore, the use of Pb is now being regulated on a global scale, and there is a strong demand in the industry using sliding parts for a sliding material not containing Pb.
Sliding materials which do not contain Pb have Cu as a main component to which Sn, Ag, Bi, Ni, Fe, Al, Mn, Co, Zn, Si, P, and the like are added. Recently, there have been many proposals of copper based sliding materials which are alloys of Cu, Sn, and Bi. See JP H10-330868A, JP 2001-81523A, JP 2001-220630A, and JP 2002-285262A.
Conventionally, these copper based sliding materials were sintered alloys of a Cu—Sn—Bi alloy powder or sintered alloys of a Cu—Sn based alloy powder mixed with a Bi powder. Bi has the same action as Pb in conventional lead bronze, namely, Bi spreads as a thin layer on the surface of a Cu—Sn alloy matrix acts as a lubricating oil to improve sliding properties.
As shown in FIG. 1, a conventional sliding material of this type (referred to below as a Bi-containing sliding material) comprised a sintered alloy layer 2 of a Cu—Sn—Bi alloy formed on a backing plate 1. The structure of a conventional Bi-containing sliding material is a structure in which a Bi phase 4 is dispersed in a Cu alloy phase 3. The liquidus temperature of the Bi phase is at least 200° C.
A brief explanation will be given of a swash plate for a compressor as an example of a sliding part which uses a sliding material. As shown in FIG. 2, which illustrates a portion of a compressor, a piston 12 is installed inside a cylinder 11 of a compressor 10 so as to reciprocate in the directions shown by arrows A. A pair of shoes 13, 13 is rotatably installed at the center of the piston 12. The pair of shoes 13, 13 sandwiches a swash plate 14 in a sloping state. The swash plate 14 is slopingly mounted on a shaft 15 installed in the vicinity of the cylinder 11.
Sliding materials 16, 16 are bonded to both sides of the swash plate 14. When the shaft 15 rotates, the swash plate 14 rotates while oscillating to the left and right with respect to the piston 12. The sliding materials 16, 16 on both sides of the rotating swash plate 14 slide with respect to the shoes 13, 13 installed in the piston 12, and the piston 12 reciprocates in the direction of arrow A. As a result of the reciprocating movement of the piston 12, a refrigerant gas in piston chambers on its left and right sides is compressed and sent to an unillustrated condenser.
FIG. 3 is a schematic perspective view of the swash plate. As stated above, the swash plate 14 has sliding materials 16, 16 bonded to both sides of a disk-shaped backing plate 17. In order to slopingly mount the swash plate 14 on a shaft, a mounting hole 18 is formed at its center, and a plurality of screw holes 19 is provided in its periphery to secure it to a shaft. If the sliding materials extend up to the location of the screw holes, they will interfere with mounting of the swash plate on a shaft with screws. The sliding materials 16 therefor are provided only on the outer side of the screw holes 19, i.e., in an annular shape excluding the center of the swash plate.